Flowers, Pollination & Ocean Dispersal In Coral Trees (Erythrina)

Depending on the botanical authority, there are 112 to possibly 116 different species of coral trees distributed throughout the world. Erythrina flowers are commonly pollinated by perching birds (passerines) and hummingbirds. Species that are pollinated by perching birds typically have gaping corollas in which the upper banner or standard is ovate or obovate and widely separated from the wing and keel petals which enclosed the stamens. The relatively short bills of these birds can easily reach the liquid nectar at the base of the banner. During feeding, pollen from the exerted stamens is transferred to the bird's head and breast. Compared with hummingbird flowers, the nectar in passerine flowers has a lower sugar content and higher amino acid concentration. Passerine pollination is commonly found in Old World species of Erythrina in Africa and in about a dozen New World species. Hummingbird flowers are typically elongate and tubular, with the standard or banner elongated and the keel and wing petals reduced. The long, probing bill of the hummingbird is well-adapted for reaching the liquid nectar reward at the base of the petals deep inside the tubular corolla. These flowers have relatively low concentrations of amino acids and a high concentration of sucrose-rich nectar to satisfy the energy requirements and high metabolic rates of hummingbirds. They are typically found in New World species of Erythrina in the Americas.

Kafferboom: One Of The World's Most Beautiful Seeds

Kafferboom (Erythrina caffra), a large, thorny coral tree up to 70 feet tall (21 m) native to coastal forests and streams of the eastern Cape region of South Africa. Like other species of Erythrina, the leaves are trifoliate with three separate leaflets. The flowers are shorter and broader than other species of coral trees, and they have widely separated (gaping) petals and conspicuous exerted stamens. Unlike the narrow, tubular, hummingbird flowers of New World coral trees, the kafferboom is pollinated by perching birds (passerines). The pods are long and slender with several bright red seeds. In South Africa, native people string the seeds into colorful necklaces. Children also collect the seeds which are known locally as "lucky beans." The seeds contain the alkaloid beta-erythroidine, a curare-like poison that paralyzes the respiratory system by blocking acetylcholine receptor sites at the synaptic junctions. This beautiful flowering tree is commonly cultivated in San Diego County, California. The sample in photo was collected at the Palomar College arboretum.

A dissected flower of Erythrina crista-galli showing all the major perianth segments removed from their attachment inside the calyx (A). The five petals consist of one large, oval banner or standard (B), two elongate keel petals (C) that are fused together enclosing the stamens, and two reduced wings (D). Nine stamen filaments (E) are united into a sheath that surrounds the pistil. One stamen filament is separate from the fused nine, a condition referred to as diadelphous. A drop of nectar is secreted at the base of the petals inside the calyx. At maturity the banner is widely separated from the keel petals, thus making the nectar fluid at the base of the petals (within the calyx) readily available to short-billed perching birds.

The cockspur coral tree (Erythrina crista-galli) native to South America. Unlike most of the elongate, tubular, hummingbird-pollinated coral trees of the New World, this species is pollinated by perching birds. Note the gaping corolla with a large, oval banner petal widely separated from the two narrow keel petals that enclosed the stamens. Two greatly reduced wing petals (barely discernible in this photo) occur at the base of the keel. In contrast to many other species with bright red seeds, the seeds of the cockspur coral tree are dark brown.

The bright red, tubular flowers of Erythrina flabelliformis are typically pollinated by hummingbirds. This interesting shrub is native to southern Arizona and New Mexico, extending southward into mainland Mexico and peninsular Baja California.

The bright red, tubular flowers of Erythrina flabelliformis are typically pollinated by hummingbirds. This interesting shrub is native to southern Arizona and New Mexico, extending southward into mainland Mexico and peninsular Baja California.

Pollination Of Erythrina Flowers By Hummingbirds

A hummingbird sipping sugary nectar from the tubular flowers of the Mexican coral tree (Erythrina coralloides). As the hummingbird feeds, pollen adheres to its head and bill is transferred from one flower to another. If the pollen is transferred between flowers of different trees, then cross pollination is accomplished. The tree supplies the hummingbird with sucrose-rich nectar for this valuable pollination service.

A hummingbird sipping sugary nectar from the tubular flowers of the Mexican coral tree (Erythrina coralloides). As the hummingbird feeds, pollen adheres to its head and bill is transferred from one flower to another. If the pollen is transferred between flowers of different trees, then cross pollination is accomplished. The tree supplies the hummingbird with sucrose-rich nectar for this valuable pollination service.

Ocean Dispersal Of Coral Trees

Erythrina is such an extraordinary genus of flowering plants that
four international botanical symposia were devoted to it during the
1970s and 1980s. Some of the world's leading botanists discussed a wide
range of topics, from the taxonomy and pollination of these trees, to
their remarkable geography and dispersal. A number of articles from the
symposia were published in several scholarly journals, including Lloydia
Volumes 37 & 40 (1977), Annals of the Missouri Botanical Garden Vol. 66
(1979) and Allertonia Vol. 3 (1982). Erythrina flowers are pollinated
by perching birds (passerines) or hummingbirds, depending on the
species. Flowers pollinated by perching birds typically have gaping
corollas in which the upper banner is widely separated from the wing and
keel petals, while hummingbird flowers typically have slender, tubular
corollas.

The exact mechanism for dispersal of Erythrina species throughout
all the major continents of the world is an enigma. Over 100 species
are distributed in the following regions: Mexico (27), Central America
(25), South America (21), West Indies (9), Tropical Africa (26), South
Africa (5), Continental Asia (6), Melanesia and South Pacific (6), and
Australia (2). One additional species (E. fusca) occurs on all the
continents except Africa, and three additional species are native to the
continental United States and the Hawaiian Islands. Since a few species
occur in more than one major continental group, the grand total is 116.

A population of native wiliwili trees (Erythrina sandwicensis) mixed with a tropical American mesquite (Prosopis pallida) on the arid, west side of Maui. Vast acres of the dry, leeward sides of the Hawaiian islands are covered by this mesquite, all originating from a seed planted in Honolulu in the early 1800s.

But why are populations of coral trees so widespread throughout the
world? Although the distribution of some ancient plant groups, such as
the cycads, correlates rather nicely with continental drift, the time
frame when all the continents were joined together is much too early for
Erythrina. More plausible theories for the intercontinental dispersal
of coral trees include seed dispersal by ocean currents, rafting, and
perhaps migratory birds. Although most species have seeds that sink in
water, there are some with buoyant seeds that may have drifted or rafted
to some of the Polynesian Islands. According to Bob Gun and John Dennis
(World Guide to Tropical Drift Seeds and Fruits, 1976), the buoyancy of
Erythrina seeds is due to lightweight cotyledonary tissue. E. Charles
Nelson (Sea Beans and Nickar Nuts, 2000) states that coral beans are
true long-distance drift seeds which can remain buoyant in salt water
for at least one year. He documents several coral bean collections from
beaches of the British Isles. It is not surprising that the most widely
distributed of all coral trees (E. fusca) has buoyant seeds. Another
species with buoyant seeds (E. variegata) occurs in Asia, Polynesia and
Australia. One remarkable observation by the naturalist H.B. Guppy
(Plant Dispersal, 1906) describes abundant seeds of an unknown Erythrina
floating off the coast of Ecuador. Because of their small size they
must have been a colorful, red-seeded species present in large numbers.
It is interesting to note that E. velutina occurs on the Galapagos
Islands 600 miles off the coast of Ecuador, where it is pollinated by
finches. More than half of the known species have bright red seeds that
may have also been dispersed by birds or mammals. In fact, the shiny
red seeds are collected by people and used for necklaces, earrings,
good-luck charms, and rosaries in Africa and Caribbean Islands. With
thick, impervious seed coats, Erythrina seeds could pass through the
digestive tract of large mammals and birds. The fact that wild
populations of coral trees are represented on practically all major
continents and many islands is truly remarkable.

Buoyant seeds of Erythrina variegata floating in a lava tide pool on the island of Hawaii. Seeds such as these are capable of drifting to distant islands of the tropical Pacific.

An interesting coral tree endemic to dry, leeward slopes of all the
main Hawaiian islands is called wiliwili (E. sandwicensis). Although
the flowers are usually red, some of the populations have color
variations of red to orange and white. This Hawaiian endemic belongs to
a group of closely related passerine species in the section Erythraster,
including E. tahitensis, endemic to Tahiti; E. variegata, a widespread
species in tropical Asia and the South Pacific; and E. velutina, another
widespread species in northern South America, the West Indies and the
Galapagos Islands. Although the seeds of wiliwili sink in water, it may
have originated from an ancestral species with buoyant seeds that
reached these islands many thousands of years ago. In fact, a related
tropical Asian species (E. variegata), which is commonly planted as a
street tree in Hawaii, has seeds that float in water. The seeds of this
species sometimes show up along Hawaiian beaches and tidal waters;
however, they are usually of local origin. Trees of E. variegata are
fairly easy to identify because the pods are longer than most cultivated
coral trees, often up to 12 inches or more in length.

Wiliwili trees (Erythrina sandwicensis) along the southern, leeward side of Maui produce pinkish-orange flowers, although this same area of ancient lava flows also has trees with greenish-white blossoms. Like other populations of this species, the seeds are bright red.

In contrast with most red-flowered, cultivated coral trees, populations of wiliwili (Erythrina sandwicensis) along the south side of Maui produce greenish-white and pinkish-orange blossoms.

According to Sherwin Carlquist (Hawaii: A Natural History, 1980),
the seeds of wiliwili (Erythrina sandwicensis) have gradually lost their
ability to float in seawater. Although the reasons are not identical,
Carlquist compares this with the loss of flight in certain insects and
birds. "If a plant shifts its ecological preference, it will tend to
lose contact with the agent responsible for its dispersal." Carlquist's
hypothesis does not apply to the three species of sea beans on the
Hawaiian Islands, including Mucuna gigantea, M. sloanei and Dioclea
wilsonii (D. violacea), which appear to have arrived on these islands
naturally and have retained their buoyancy and ecological preference.
According to the second edition of Manual of the Flowering Plants of
Hawaii by W. L. Wagner, D.R. Herbst and S. H. Sohmer (1999), all three
lianas appear to be indigenous, and one variety of M. sloanei (var.
persericea) is endemic to Maui. Unlike populations of Erythrina on
these islands, the sea bean species appear to have maintained their
ecological preference for wet, lowland forests where they sprawl over
rocks, shrubs and trees along streams and near the ocean.

Seeds of the Asian coral tree (Erythrina variegata) are buoyant in seawater and may have drifted to distant shores of the tropical Pacific. In fact, this species (or its progenitor) may have given rise to several species of endemic coral trees in the tropical Pacific region.

Although natural populations of wiliwili typically occur on the
drier, leeward slopes of the islands, Carlquist postulates that they
probably arrived by drifting (either buoyant seeds or floating limbs
bearing seed pods) because of their proximity to the ocean. Perhaps E.
velutina arrived on the Galapagos Islands by a similar mechanism. In
fact, mainland iguanas probably rafted to the Galapagos Islands and gave
rise to the present-day populations of marine iguanas. Based on the
speed of the currents and local winds, it has been estimated that a raft
from Ecuador could reach these islands in about two weeks. It is
interesting to note that the Hawaiian wiliwili has a soft, light wood
that was (and still is) highly prized for the outriggers of traditional
canoes. It was also used for fishnet floats and surfboards.

This dried limb of wiliwili (Erythrina sandwicensis) weighs only ten ounces (284 g). Althought it is quite strong, it has the buoyancy of balsa wood and was commonly used for ouriggers on traditional Hawaiian canoes.